Myocardial infarction is one of the leading causes of death in the United States. Approximately 5 million individuals experiencing chest pain are evaluated every year in hospitals throughout the United States, however, less than 30%, of these individuals are subsequently found to have had a myocardial infarction. The accurate and rapid diagnosis of myocardial infarction is important both for the patient suffering a myocardial infarction and for the health care system which can minimize the costs incurred by rapidly identifying individuals who do need treatment.
The diagnosis of myocardial infarction is usually performed in the emergency department of a hospital. An individual having the symptoms of myocardial infarction is treated in different ways depending on the obviousness of the condition. Generally, an electrocardiogram is given to assess the condition of the heart; however, approximately 50% of patients experiencing myocardial infarction have a non-diagnostic electrocardiogram. The physician is then faced with a problem of diagnosing and treating the patient suspected of having a myocardial infarction. Thus, diagnosis and treatment is difficult for patients with a suspected myocardial infarction who have non-diagnostic electrocardiograms.
The World Health Organization (WHO) has instituted guidelines for diagnosing myocardial infarction which state that an individual must exhibit two-of-the-three following criteria: 1) have chest pain or a history of cardiac disease; 2) a diagnostic electrocardiogram; and, 3) elevated creatine kinase (CK) or creatine kinase MB isoenzyme (CKMB). Thus, for the 50% of the individuals who are presented to hospitals for a suspected myocardial infarction and who have a non-diagnostic electrocardiogram, the physician must rely on symptoms of chest pain and an elevated CK or CKMB to diagnose a myocardial infarction.
The assay of CK or CKMB is generally performed in hospital laboratories using sophisticated instrumentation. The assays include enzyme assays and immunoassays which detect the activity or mass of CK or CKMB present in blood samples.
During a myocardial infarction, heart muscle cells die and release their contents to the blood stream. Troponin I is one such muscle content that is released into the blood after a myocardial infarction. In addition, CKMB is released into the blood stream after a myocardial infarction. CKMB becomes elevated above an otherwise nominal value and can be diagnostic for myocardial infarction. The specificity of CKMB for diagnosing myocardial infarction is not 100% because another source of CKMB in the body is skeletal muscle. Since the mass of skeletal muscle in the body far exceeds the mass of cardiac muscle, through the normal catabolic turnover of skeletal muscle cells, the blood concentration of CKMB in healthy individuals will vary. In general, the concentration of CKMB which may be indicative of myocardial infarction is above 5-7 ng/ml (Circulation 87, 1542-1550 (1993), Clin. Chem. 39, 1725-1728 (1993)). The CKMB concentration of individuals who have skeletal muscle injury or who have exercised has been reported to be elevated above 9 ng/ml (Clin. Chem. 38, 2396-2400 (1992)). Therefore, the problem of specificity when using CKMB as a marker for myocardial infarction has prompted the search for other more specific markers which are released only from damaged heart muscle.
Troponin I and troponin T have recently been shown to be more specific than CKMB for diagnosing myocardial infarction (Circulation 83, 902-912 (1991), Clin. Chem. 40, 1291-1295 (1994). Although troponin T has some disadvantages as a marker because it is elevated in patients experiencing renal disease (Clin. Chem. 41, 312-317 (1995)), the inventive methods herein disclose the successful use of troponin T as a diagnostic marker. The use of troponin I as a diagnostic marker for myocardial infarction also appears to meet many of the clinical requirements (Clin. Chem. 40, 1291-1295 (1994), Clin. Chem. 41, 312-317 (1995)).
The troponin complex in muscle is comprised of troponin I, C and T. These troponin components exist as various tissue specific isoforms. Troponin C exists as two isoforms, one from cardiac and slow-twitch muscle and one from fast-twitch muscle. Troponin I and T are expressed as different isoforms in slow-twitch, fast-twitch and cardiac muscle (Biochem. J. 171, 251-259 (1978), J. Biol. Chem. 265, 21247-21253 (1990), Hum. Genet. 88, 101-104 (1991), Circul. Res. 69, 1226-1233 (1991)). The unique cardiac isoforms of troponin I and T allow them to be distinguished immunologically from the other troponins of skeletal muscle. Therefore, the release into the blood of troponin I and T from damaged heart muscle has been related to cases of unstable angina and myocardial infarction. The prior art, however, has not addressed other forms of troponin I and T in blood.
The troponin complex in muscle is tightly bound to the contractile apparatus. Approximately 6% of the troponin T in cardiac tissue exists as an unbound protein in the cytoplasm and it is believed that this pool of troponin T is released from damaged muscle (Am. J. Cardiol. 67, 1360-1367 (1991)).
The conformations of troponin I, T and C change upon binding when forming binary and ternary complexes (Biochemistry 33, 12800-12806 (1994), J. Biol. Chem. 254, 350-355 (1979), Ann. Rev. Biophys. Biophys. Chem. 16, 535-559 (1987)). An understanding of the conformational changes of troponin I and troponin T and the heterogeneity of the proteins in the blood is critical for the development of accurate diagnostic procedures for measuring troponin I and troponin T concentrations. In addition, troponin I is reported to be unstable in blood (Direction Insert for Troponin I Immunoassay, Sanofi/ERIA Diagnostics Pasteur, Marnes la Coquette, France), and the mechanisms responsible for the instability have not been understood. This invention addresses these problems and provides for stable troponin I and T compositions which are useful in immunoassays.
The state of troponin I and troponin T released from muscle after a myocardial infarction was previously investigated in PCT publication WO 96/33415, which is incorporated herein by reference in its entirety, including all figures and drawings. Methods for the selection of antibodies and their use in immunoassays for troponin I and troponin T and complexes of these proteins are also described in the WO 96/33415 publication. Troponin I and T, along with troponin C, exist in both cardiac and skeletal muscle mainly as a ternary complex. In the muscle, the troponin complex is bound to tropomyosin which is, in turn, bound to the actin comprising the thin filaments.